Territory level breeding phenology temperature plasticity of the Common Redstart (Phoenicurus phoenicurus) and two sympatric Paridae

Based on individual nest site temperature measurements at an upland study area in Central England, this study demonstrates spatial temperature plasticity in breeding timing at the local territory level by which Common Redstart (Phoenicurus phoenicurus) and two sympatric Paridae can track food resources during breeding, and similar interannual temperature-related plasticity. The quantitative plasticity estimates are in the range of -4 to -7 days/°C, at the upper end of the range of values previously reported, or steeper. In addition, the study provides evidence of phenological constraints on Redstart breeding related to the timing of return migration in the spring. Redstarts forego otherwise suitable breeding habitat employed by sympatric residents where spring has advanced beyond a certain stage. At the territory level at a fine scale, they select cooler breeding sites which are less phenologically advanced when they return, avoiding warmer lower lying and south facing areas where breeding activity by residents has already commenced. Redstart fledging success was high with chicks fledged from 87% of eggs hatched, excluding predations and similar losses, suggesting that trophic asynchrony has little impact on breeding productivity. These observations of preferential settlement are set against the background of a decline in this species across the southerly edge of its UK range. They suggest that rather than trophic asynchrony in philopatric populations driving declines at the warmer edge of the range under climate change, a more active mechanism for range shift may apply. It appears that settlement is influenced by climatic conditions, whereby individuals select breeding territories based on phenological criteria in the areas they encounter, maintaining a climatic niche. Finally, the discrepancies between the quantitative estimates for plasticity determined in this study and others in the literature raise questions about the reliability of the regression models employed that deserve further attention.